3.2. Shared State Cache

+3.2. Shared State Cache

+ By design, the OpenEmbedded build system builds everything from scratch unless + BitBake can determine that parts don't need to be rebuilt. + Fundamentally, building from scratch is attractive as it means all parts are + built fresh and there is no possibility of stale data causing problems. + When developers hit problems, they typically default back to building from scratch + so they know the state of things from the start. +

+ Building an image from scratch is both an advantage and a disadvantage to the process. + As mentioned in the previous paragraph, building from scratch ensures that + everything is current and starts from a known state. + However, building from scratch also takes much longer as it generally means + rebuilding things that don't necessarily need rebuilt. +

+ The Yocto Project implements shared state code that supports incremental builds. + The implementation of the shared state code answers the following questions that + were fundamental roadblocks within the OpenEmbedded incremental build support system: +

What pieces of the system have changed and what pieces have not changed?
How are changed pieces of software removed and replaced?
How are pre-built components that don't need to be rebuilt from scratch + used when they are available?

+ For the first question, the build system detects changes in the "inputs" to a given task by + creating a checksum (or signature) of the task's inputs. + If the checksum changes, the system assumes the inputs have changed and the task needs to be + rerun. + For the second question, the shared state (sstate) code tracks which tasks add which output + to the build process. + This means the output from a given task can be removed, upgraded or otherwise manipulated. + The third question is partly addressed by the solution for the second question + assuming the build system can fetch the sstate objects from remote locations and + install them if they are deemed to be valid. +

+ The rest of this section goes into detail about the overall incremental build + architecture, the checksums (signatures), shared state, and some tips and tricks. +